The present invention is especially directed to thermoforming machines of the type in which a pair of mold carrying platens are cyclically driven in movement between open and closed positions to thermoform parts on a mass production basis. The characteristics of the articles or of the thermoplastic material from which the articles are being formed can vary, and some materials require a greater length of time in the forming stage than do others. Because these machines are designed for cyclic operation, it is usually difficult and sometimes impossible to change the time duration of one stage of the cycle without proportionately affecting the time duration of other stages of the cycle. Thus, merely slowing down the main drive to extend the time period of the mold closing stage or the mold closed stage of the cycle is usually impractical. In some cases it has been proposed to stop the main drive for a given period of time at some point in the cycle, however, this concept has proved less than satisfactory in practice due to the difficulties in obtaining drive components, particularly clutches, which will stand up under the mechanical loads imposed by such arrangements.
The present invention is especially directed to a positive mechanical linkage type platen shifting arrangement which enables the time during which the mold platens are closing or are held in their closed position to be adjustably varied independently of each other and without any consequent variations of the time duration of other stages of the cycle and without imposing any undue loading on any components of the drive system.